The “airlay” technique basically consists of dispersing individual fibers in a chamber and projecting them onto a moving receptive surface by means of a high-speed air flow; said receptive surface is permeable to air and allows said non-woven material to be formed and conveyed. The term “non-woven” in this text designates the web of fibers formed by the “airlay” technique, even when this web has not undergone any special bonding technique.
Such an “airlay” technique is known particularly from documents U.S. Pat. No. 4,097,965, EP 0 093 585 and FR 2 824 082.
In these three documents, the means of producing an air flow inside the dispersion chamber that allows the fibers to disperse within the chamber and be projected onto the forming and conveying surface consist particularly of vacuum means located below the forming and conveying surface of the non-woven material which is permeable to air.
In document U.S. Pat. No. 4,097,965, the wall downstream from the dispersion chamber is a plate whose lower edge is applied to the surface of the non-woven material coming out of said chamber, with the vacuum tank mounted over the whole surface, which extends perpendicular to the lower edge of the wall upstream and the lower edge of the wall downstream from the dispersion chamber. In this text, the terms “downstream” and “upstream” are defined in relation to the direction in which the forming and conveying surface of the non-woven material moves.
According to the applicant, contact between the lower edge of the downstream wall of the dispersion chamber and the surface fibers of the non-woven material generates friction that can cause irregularities in the non-woven material, especially if the forming and conveying surface of the non-woven material moves at high speed.
In document EP 0 093 585, there is a transverse cylinder at the output of the dispersion chamber that is set in rotation in the direction in which the non-woven material moves. The rotation of this cylinder, which constitutes in some way the lower edge of the wall downstream from the dispersion chamber, makes it possible to limit the friction and hence accompany the surface fibers of the non-woven material when they come out of the dispersion chamber. However, according to the applicant, if you increase the speed at which the non-woven material moves on the forming and conveying surface and, consequently, the speed of rotation of the transverse cylinder, parasitic air flows are produced that interfere with the homogeneity of the non-woven material when it passes under the transverse cylinder.
In document FR 2 824 082, the lower part of the front wall of the dispersion chamber is porous, and the profile of said lower part is preferably curved approximately like the arc of a circle. This prevents the production of parasitic air flows caused by the rapid rotation of the transverse cylinder. However, in operation, the thin microperforated sheet metal that constitutes the lower part of the wall downstream from the dispersion chamber exerts a low compressive force on the non-woven material that slightly compresses it. This prevents the vacuum flow produced by the vacuum tank from causing an incoming air flow that would penetrate inside of the dispersion chamber, passing between the lower edge of the downstream wall and the upper end of the forming and conveying surface of the non-woven material; such an air flow is detrimental to the quality of said non-woven material.
However, according to the applicant, this contact between the thin microperforated sheet metal and the surface fibers of the non-woven coming out of the dispersion chamber causes friction that can deform the non-woven material and produce irregularities on it, and even more so the higher the speed at which the forming and conveying surface of the non-woven material moves.
In document FR 2 824 082, the lower porous part of the front wall of the dispersion chamber can also be comprised of a porous rotary cylinder, particularly a microperforated cylinder. This embodiment makes it possible to avoid friction when the cylinder is driven at a peripheral speed equal to the speed at which the forming and conveying surface of the non-woven material moves. However, some parasitic air play may subsist, even if it is not as much as in document EP 0 093 585.